The present invention relates to a composite optical element comprising a glass base and a resin layer thereon, and more particularly, to a compact composite optical element adapted for use in a camera.
An optical element or lens used in a camera has a rectangular effective range for guiding light to a film according to the shape of a frame formed on the film. Based on this fact, a compact camera lens useful for camera is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-177105. The compact lens is made by partially removing outside of the rectangular effective range from a conventional circular composite lens. The resulting lens has a shape asymmetrical in rotation about its central axis, that is, a noncircular shape. An additional space obtained by the removal can be utilized for the arrangement of other camera components.
The composite lens comprises a glass base and a resin layer thereon. The glass base and the resin layer are cut together as an unnecessary portion of the lens is eliminated for compactness. Cutting the glass base produces a large number of burrs or fine chips, from which fine cracks extend individually inward.
In general, resin has a thermal linear expansion coefficient higher than that of glass, so that resin is subject to a greater dimensional change with temperature than glass is. Thus, if the temperature of the lens which is composed of the glass base and the resin layer thereon changes, the resin layer undergoes a greater deformation than the glass base does, so that stress is generated between the two. For example, a simulation of the finite element method tells that the stress is the heaviest at an end portion of the resin layer.
A cut surface of the lens constitutes the end portion of the resin layer, where a large number of fine chips are formed on the glass base. Therefore, the fine chips are subjected to a relatively heavy stress. The stress sometimes enlarges the cracks that extend from the fine chips, thereby causing the glass base to be damaged. In such a case, the lens cannot serves as a useful component any longer.